FMF - Friends of Minerals Forum, discussion and message board

[John S. White]

I agree with Peter that "pseudo-cubic" is more accurate ( https://www.mineral-forum.com/message-board/viewtopic.php?p=3654#3654 ) and I disagree with Pete Richards over referring to them as opaque ( https://www.mineral-forum.com/message-board/viewtopic.php?p=3079#3079 ). I was able to obtain one of these from Sr. Pastor for which I am very grateful (I hope I am using the correct name) and it is quite translucent. In fact, the inclusions can be readily seen so the crystals themselves are actually transparent. According to Jordi the inclusions are riebeckite-magnesioriebeckite. Also, the crystals on my specimen are all perfect little Cumberland habit crystals.

The term opaque is, in my opinion, one of the most often miss-used terms in mineralogy. I have written a column on this subject which was published in Rocks & Minerals and I will see if it can be reproduced here.

[Pete Modreski]

I'd like to add my two cents, too, that those photos of blue quartz crystals are really superb, Jose Ramon, thank you for posting them; I think they are the best such crystals that I've seen pictured.

Taking note of John White's comment about them not being truly "opaque", John has taken me to task in the past too, for writing about things being "opaque" which are not truly so. I'll just add my little comment (pending his posting a copy of his column) that I think "opaque", in common usage, can be a matter of degree. The truly "opaque" minerals--metals and semimetallic ones--are opaque even in thin section, but even some of these--for example, hematite--can transmit some light and show color in very thin fragments or tiny crystals (and yes, I know that John would now add, that those therefore by definition are not truly opaque!). But a common usage is probably to refer to anything that, in perhaps in inch or centimeter-size pieces, doesn't obviously transmit very much light or enough light to see at least a blurry distorted image through it, is "opaque", though this isn't the true, valid use of the term. [Not trying to start an argument, just making comments!]

cheers,
Pete

[John S. White]

While still waiting for my column to be available for posting, let me say that I do not disagree with anything that Pete wrote. In the absence of standards, I just feel that the term opaque should be used with more care. There are many minerals that simply look opaque and, in general, no one would challenge their being described as such. Among these are many of the native elements, of course, and a large number of black minerals such as hematite, ferrocolumbite (columbite-(Fe)), hornblende, etc. On the other hand, I can't recall ever seeing a truly opaque quartz, so my objecting antennae shoot up when I see quartz described as opaque. Of course, this gets tricky, because are we talking about the quartz itself or the almost opaque inclusions within it?

More to come, please stay tuned to this station.

[John S. White]

I realize that there is a fine line or a subtle distinction between minerals that are truly opaque and those that simply appear opaque. What I would suggest is that before using the term one should attempt to place a strong light behind the sample to see if any light is transmitted before calling it opaque. Of course one can also use the modifying term "apparently opaque."